As a Magnetic Flux Leakage (MFL) tool traverses along a section of pipe, the pipe wall is locally magnetically saturated. At locations where a defect exists, magnetic flux intensity changes. This change is detected by a set of sensors as the tool passes down the pipeline and is recorded on a data logger. This paper describes Finite Element (FE) modelling of a spherical defect located within the outside of the pipe wall. Two meshes are created: a mesh of an axial MFL tool and a mesh of the pipe (containing the defect). To perform a dynamic, time-transient analysis, the meshes are joined together using a Lagrangian interface, set in motion relative to each other and the magnetic field calculated for several time steps. The corresponding magnetic signature of the defect can be calculated. Two pipe wall sizes are considered (a thick 0.625 inch pipe wall and a thin 0.375 inch pipe wall) for a 30 inches pipe Outer Diameter (OD), with defects positioned directly above the line travelled by a set of the MFL tool’s magnetic poles and offset between the magnetic poles. Corresponding signal-to-background field analysis (the magnetic bias ratio), for a tool moving at a speed of 1mph, reveals the thicker pipe gives the largest ratio, with a central defect location giving a better ratio than an offset defect. The analysis can be used to predict the best sensor positions, the magnetic bias ratio and the optimal circumferential sensor spacing. The method can be used to investigate a variety of defects and to gauge the performance and limitations of the axial MFL technique. It should be noted that FE methods do not presently incorporate the full complexity of the magnetic material properties and that realistic, complicated defect geometries will be computationally expensive. However, the results presented give a theoretical indication of typical magnetic signatures that may be obtained for spherical defects using the axial MFL technique.

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